Differential protective relay apparatus

ABSTRACT

A differential protective relay apparatus is switched over between a current differential mode and a voltage differential mode automatically by both even-order harmonic components of an imput applied to a differential circuit and a voltage across an impedance of the differential circuit, thereby to cover extensive fault modes while retaining high minimum pickup sensitivity, furthermore, devised against excessive fault voltages in the voltage differential mode, and, consequently, advantageous in insulation withstand capability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a differential protective relayapparatus which differentially protects a multiple-branch bus bar havinga plurality of terminals through current transformers each provided onthe terminals respectively.

2. Description of the Prior Art

FIG. 1 shows a connection diagram of a power system to which adifferential protective relay apparatus is applied. In the figure,indicated by 0 is a bus bar, 1 to n are terminals of the bus bar 0, 11to 1n are current transformers (will be termed CTs hereinafter) providedfor the terminals 1 to n, 20 is a differential circuit which is aparallel connection of the secondary circuits of the CT11 to CT1n, andZ_(D) is the impedance of a differential relay 87 connected betweenterminals 20-1 and 20-2 of the differential circuit 20.

Generally, differential protective relay apparatus are categorized intotwo types of a high-impedance differential system and low-impedancedifferential system depending on the selected value of a high or lowimpedance Z_(D) between the terminals of the differential circuit 20.

In the former high-impedance differential system, the differentialcircuit is shunted by a relatively high impedance Z_(D), and thereforeit takes a little shunted current components from the currenttransformers CT11 to CT1n with differential connection and a littletransferred energy from them. Accordingly, when currents flow in thesame direction toward the differential circuit 20 on an internal fault,a relatively high voltage appears across the terminals of thedifferential relay 87 of the differential circuit 20. On the other hand,in case currents circulate through the CT11 to CT1n in differentialconnection on an external fault, voltage drops across the lead wireresistances of the secondary circuits of the CT11 to CT1n indifferential connection are applied to the excitation impedance of theexternal fault current flow-out CT, and the terminal voltage does notexceed a certain voltage value determined by the CT excitationcharacteristics.

The low-impedance differential system introduces a great amount ofshunted current components from the CT11 to CT1n with differentialconnection to the differential relay 87 of impedance Z_(D), and most ofenergy is transferred to the differential circuit. Accordingly, aninternal fault does not result in the induction of a high voltage acrossthe differential relay 87 of the differential circuit 20. On the otherhand, upon application of the voltage drops across lead wire resistancesof the secondary circuits of the CT11 to CT1n at an external fault, theimpedance Z_(D) of the differential circuit 20 becomes equal to or lowerthan the secondary excitation impedance of CT1n of the external faultcurrent flow-out terminal, resulting possibly greater flow-in current tothe differential relay 87 of the differential circuit 20. On thisaccount, the low impedance differential system is prone to malfunctionon an external fault current.

The former high-impedance differential system will further be examinedin the following. Generally, assuming that R_(D) is a resistance of adifferential circuit and R₂ is a total resistance the secondary circuitsof CTs (secondary winding resistance R_(S) plus secondary lead wireresistance R_(L) of CTs), the maximum external fault current I_(FE) maxcauses an apparent differential circuit current I_(D) and differentialcircuit voltage V_(D) as follows. ##EQU1##

If R_(D) >>R₂, the differential circuit voltage becomes:

    V.sub.D ≦R.sub.2 I.sub.FE max ...                   (1.3)

and it does not exceed a certain voltage value.

On an internal fault, the minimum internal fault pick-up current isgiven in terms of the voltage V_(S) appearing across the impedanceZ_(D), the secondary excitation current I_(ex) (V_(S)) for the appliedvoltage V_(S), and the number n of terminals connected to the bus bars,as follows. ##EQU2##

The conventional high-impedance differential protective relay schemeshave the foregoing arrangement and operation, involve the followingproblem to be overcome. Although, with the intention of preventing themalfunctioning, the differential relay can be set to a value lower thanthat given by the formula (1.3) on external faults, the minimum pick-upcurrent on internal faults is limited to the value given by the formula(1.4) as long as the relay being set in compliance with the formula(1.3). Namely, when a large number of terminals are connected to the busbar O and the internal fault current is small, the minimum pick-upsensitivity of fault detection adversely varies by both the secondaryexcitation characteristics I_(ex) -V_(ex) of the CTs and the number n ofterminals.

SUMMARY OF THE INVENTION

The present invention is intended to solve the foregoing prior artproblem, and its prime object is to provide a differential protectiverelay apparatus which allows the setting with wide latitude so that evena small internal fault current can surely be detected.

Another object of this invention is to prevent the insulation of the CTsecondary circuit from being jeopardized.

In order to achieve the above objects, a bus bar differential protectiverelay apparatus according to one aspect of this invention comprises afirst switching over circuit which switches an impedance of adifferential circuit on the basis of a functional value between afundamental component and even-order harmonic components in an input ofthe differential circuit 20, a plurality of voltage detecting elementsfor detecting a terminal voltage of the differential circuit and aterminal voltage of the impedance, and an interlock circuit formed ofthe voltage detecting elements, for tripping a circuit breaker.

A bus bar differential protective relay apparatus according to anotheraspect of this invention further comprises a second switching overcircuit which switches the impedance of the differential circuit inaccordance with the terminal voltage of the impedance of differentialcircuit into another impedance different from that switched by the firstswitching circuit.

These and other objects and novel features of the present invention willbecome more apparent from the following detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a connection diagram of a power system to which the presentinvention is applied;

FIG. 2 is a diagram explaining the principle of the inventive apparatusin the event of an external fault;

FIG. 3 is a diagram explaining the principle of the inventive apparatusin the event of an internal fault;

FIG. 4 is a graph explaining fundamental characteristics of theinventive apparatus;

FIG. 5 is a block diagram showing an inventive differential relay;

FIG. 6 is a block diagram showing a harmonic ratio discriminationcircuit; and

FIG. 7 is a schematic diagram of an interlock circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of this invention will be described with reference to thedrawings. First, the operational principle at the occurrence of a systemfault will be explained, separately for the cases of an external faultand internal fault.

(a) Heavy external fault

In the event of a heavy external fault, currents I₁, I₂, I_(n-1) fromthe terminals 1 to n-1 flow through the bus bar O toward a fault point Fon the terminal n, and a current I_(n) (I_(n) =I_(FE)) flows out of theterminal n, as shown in FIG. 2.

Current transformers CT11 to CT(1n-1) of the terminals 1 to n-1 haveeach a winding resistance and secondary lead resistance, with voltagedrops across the respective resistances being shown by (1, 2, 1), (1, 2,2), ..., (n-1, n, 2) as equivalent circuits in FIG. 2. Consequently, thefollowing differential circuit voltage V_(D) appears across theimpedance of the differential circuit 20, as is generally known in theart.

    V.sub.D ≦I.sub.FE (R.sub.S +R.sub.L)/N ...          (2.1)

where N is a CT winding ratio, I_(FE) is an external fault current (interms CT secondary), R_(S) is a secondary winding resistance of CT, andR_(L) is a sum of secondary lead resistances of CTs.

(b) Minimum internal fault

In the event of a fault at point F on the bus bar O, the currentdistribution can be shown by solid arrows and dashed arrows in FIG. 3.In this case, the detection sensitivity is minimum, and this value interms of CT primary I_(F1) is given by:

    I.sub.F1 /N=n·f.sub.e (V.sub.D)+g.sub.Z (V.sub.D) ... (2.2)

where i_(e) =f_(e) (V_(D)) is a function determined by the CT secondaryexcitation characteristics, i_(R) =g_(z) (V_(D)) is a functiondetermined by the impedance characteristics of the differential relay87, and n is the number of terminals.

Generally, it is not easy to alter the value of the function i_(e)=f_(e) (V_(D)), which is dependent on the CT secondary excitationcharacteristics, after manufacturing, while the impedance of thedifferential relay 87 can be altered. Although values given by thefunction of i_(R) =g_(z) (V_(D)) can be made sufficiently small usually,the CT secondary excitation current determined by the function of i_(e)=f_(e) (V_(D)) becomes not negligible in case that the differentialcircuit voltage V_(D) for internal fault detection is as high as 100volts or more, affecting the minimum pickup on internal fault in termsof CT primary. In order to avoid this influence, it is necessary to havethe impedance characteristics of i_(R) =g_(z) (V_(D)) so that thedifferential relay 87 can operate even under a lower differentialcircuit voltage V_(D), and to make a leakage component, i.e., the CTsecondary excitation current expressed by i_(e) =f_(e) (V_(D)), as smallas possible.

Next, the fundamental characteristics of the foregoing differentialprotection will be described in connection with FIG. 4. In FIG. 4, thedifferential circuit voltage V_(D) and CT secondary excitation voltageV_(ex) are plotted along the vertical axis, and the fault current I_(F)and CT secondary excitation current I_(ex) are plotted leftward andrightward, respectively, along the horizontal axis. The operatingcharacteristics of the differential relay 87 is expressed by V_(D)-I_(D), the CT secondary excitation characteristics is expressed byV_(ex) -I_(ex), and the excitation characteristics of a parallelconnection of n CTs each having the V_(ex) -I_(ex) characteristics isexpressed by V_(exn) -I_(ex).

The following describes the operation principle based on the above setupof characteristics, separately for each type of fault.

(i) Detection on internal fault

In the operating characteristics V_(D) -I_(F) of the differential relay87 shown in the left-hand part of FIG. 4, the slope section ofresistance R_(D1) of the differential circuit is the object of concern.The resistance R_(D1) of the differential circuit is selected to be of arelatively low value on the order of ohms. When a fault current I_(F)flows in and the differential voltage V_(D) reaches to the minimumpickup voltage V₁, the differential relay 87 operates. The current interms of the system primary side I_(Fl) is equal to the minimum pickupcurrent I_(Fl) min in the case of a single CT, but if there are unusedterminals of n in number each connected to a CT, the differentialvoltage is applied to the secondary of n CTs in parallel connection andthe current becomes as follows.

    I.sub.F min=I.sub.F1 +I.sub.exn.1 ...                      (2.3)

where I_(exn).1 is the CT secondary excitation current at the minimumpickup differential voltage V₁ when n CTs connected in parallel.

By choosing a low minimum pickup differential voltage V₁, the current interms of system primary side I_(F1) and I_(exn).1 for the voltage V₁become sufficiently small, and the degradation of the fault detectionsensitivity due to the parallel connection of n CTs can be under controlwithin a certain range. Namely, the gradient of the operatingcharacteristics V_(D) -I_(F) of the differential relay 87 is made smallso that differential circuit voltage V_(D) at the minimum pickup can beset small. This results in a smaller CT secondary excitation current,and the higher sensitivity performance is accomplished by operating theCTs in lower magnetic flux density. Consequently, the variation range ofminimum pickup due to the variation in the number of terminals connectedto the bus bar 0 can be suppressed below a certain safety threshold in apractical sense.

(ii) Prevention of malfunctioning at maximum external fault

Generally, when n terminals 1 to n are connected to the bus bar O, thedifferential circuit voltage V_(D) appearing across the impedance Z_(D)of the differential circuit at the occurrence of an external fault onthe terminal n, as shown in FIG. 2, is expressed by the above formula(2.1). By plotting the differential circuit voltage V_(D) equal to V₂ onthe graph of FIG. 4 to determine a V₃ with a certain marginal factorover the V₂, the differential voltage detecting elements that aredesigned to operate at the level of V₃ do not malfunction even duringthe maximum external fault current on the terminal n.

In this case, the internal fault current for producing a voltage of V₃takes a value of mI_(F1) which compares with point P₃ in FIG. 4. The CTsecondary excitation current varies between I_(ex1).3 and I_(exn).3. Asa result, the system minimum pickup current (in terms of CT secondary)I_(F), seen from the CT secondary side will have the following variationrange.

    mI.sub.F1 +I.sub.ex1.3 ≦I.sub.F '≦mI.sub.F1 +I.sub.exn.3 ... (2.4)

In conclusion, the minimum pickup varies depending on the magnitude ofthe CT secondary excitation current corresponding to the CT differentialcircuit voltage and the number of terminals.

(iii Overall operation

As mentioned previously, between the minimum internal fault and maximumexternal fault, there are two kinds of internal fault current detectedvalues, i.e., I_(F1) and mI_(F1), in correspondence to the magnitude ofthe differential circuit voltage V_(D).

In order to obtain these two operating values, the differentialprotective relay apparatus proposed previously by the applicants of thepresent invention in U.S. patent application Ser. No. 438,109, filedNov. 20, 1989, now U.S. Pat. No. 4,991,052 (hereinafter will be termedsimply former patent application) is designed to choose a properimpedance switching voltage V₀ as follows. ##EQU3## where I_(F1) =V_(O)/R_(D1).

The value of the impedance switching voltage V₀ is selected to bekI_(F1) for a fault current I_(F1) and, at the same time, to be equal tothe minimum pickup differential voltage V₁ at which a high-sensitivitydifferential current detecting element having the low impedance R_(D1)operates, or alternatively it is selected to be a proper value belowmI_(F1) corresponding to the voltage V₃. As a result, switching of thedifferential circuit takes place from impedance R_(D1) to R_(D2) at alevel of relatively low differential circuit voltage V_(D), and it has awide current range in which the differential circuit impedance takes thehigher value R_(D2).

In contrast, the present invention is intended to have a wide currentrange in which the differential circuit impedance takes the lower valueR_(D1), and this will be described in brief in the following.

The present invention intends to utilize the presence of even-orderharmonic components, such as the second harmonic, in the waveform of thedifferential circuit current (voltage) as a condition for switching thedifferential circuit impedance.

It is well known fact that in the current differential scheme with arelatively low differential circuit impedance, the CT is subjected tod.c. biased magnetization by a d.c. component of fault current duringthe medium fault current through the fault terminal in the event of anexternal fault, resulting in the occurrence of a d.c. saturation inwhich the magnetic flux in the core reaches the saturation level only onthe side of one polarity. It is also known that the excitation currentproduced by the d.c. saturation of CT richly includes even-orderharmonic components such as the second harmonic component in addition toa d.c. component, due to off-set fault current.

Accordingly, the differential circuit impedance R_(D1) should beswitched over to the higher value R_(D2) only after the CT of theterminal from which the external fault current flows out has beensaturated with the d.c. bias, and the second harmonic component andother even-order harmonic components have increased, and the magnitudeof the harmonic components or its proportion relative to that of thesinusoidal fundamental component has exceeded a predetermined value.Since the second harmonic and other even harmonic components increase asthe fundamental component of the fault current has increased, thedetection of fault can be done by detecting the second and other evenharmonic components in excess of a certain proportion level in case theeffective sinusoidal fundamental component has a large value.

Indicated by I in the left-hand part of FIG. 4 is a dual slopecharacteristics of V_(D) -I_(D), and II represents the relation betweenthe error differential voltage V_(E), which is produced across thedifferential circuit during the flowing of the external fault currentI_(F), and the fault current I_(F). The slope of the V_(E) - I_(F)characteristics does not exceed the slopes determined by thedifferential circuit impedance R_(D) and the total resistance value ofthe CT secondary circuit, and therefore malfunction does not occur up topoint P₀ ' (nI_(F1), V₀) in the figure. However, if the fault currentreaches lI_(F1), the differential circuit voltage will reach V₂, andtherefore the differential circuit impedance must have been switchedover at P₀ ' before reaching P₂. Namely, the V_(D) -I_(D)characteristics is switched over at point P₀ on the dual slopecharacteristics I-I', and the impedance of the differential circuit willtake the higher value R_(D2) in a significantly wide range.

Even if the fault current I_(F) increases, the difference V_(D)resulting from the subtraction of the even harmonic components includingthe second harmonic component (inclusive of the d.c. component) from thefundamental component in the differential circuit voltage will increaseas the fault current I_(F) does, and therefore it is not proportional tothe fault current I_(F) as of the case where there is CT saturation, butdevelops a tendency as shown by the dashed line III in FIG. 4, forexample.

In this invention, it is designed such that the switching over conditionof the differential circuit impedance is done only in response to theflowing of a large fault current lI_(F1), by setting the switching overvoltage V_(DS) as follows. ##EQU4## where I_(D1) is the fundamentalcomponent of differential current corresponding to the differentialcircuit voltage V_(D), and I_(D2), I_(D4), ..., I_(D2n) are even-orderharmonic components included in the differential current I_(D), whereinthe V_(D), I_(D), I_(D1), I_(D2), I_(D4), and so on appearing in thedifferential circuit on an external fault are functions of the CTexcitation characteristics, constants of CT secondary circuit and faultcurrent I_(F).

The CT excitation characteristics, which is a nonlinear characteristicsdue to the CT core saturation, cannot simply be formulated. An exampleof the result of analysis by using a simple model of CT characteristicsis disclosed in publication "IEEE Transactions on Power Apparatus andSystems", Vol. PAS-104, No. 3, P. 678, FIG. A.1, March 1985. Theforegoing discussion is summarized by formulas as follows. ##EQU5##Namely, it is designed such that impedance switching over for thedifferential circuit is not based on the apparent differential circuitvoltage, but the switching voltage is determined by the functionD(V_(D)) which is dependent on the hysteresis of the CT characteristics,so that the impedance of differential circuit is switched over fromR_(D1) to R_(D2) only in the case of flowing a large external faultcurrent lI_(F1)).

In the case of an internal fault, the current does not concentrate toone of CTs to cause it to be saturated, and therefore little hormoniccomponents are created. ##EQU6## Accordingly, the condition of minimumfault pickup is as follows. ##EQU7## In other words, a minimum pickuptakes place on the line I of the V_(D) -I_(F) characteristics, and theoperation accompanied by the switching over to the high impedancesection i.e., line I', on a heavy fault, is the case where the followingrelation is met; namely, at point P₃ (mI_(F1), V₃) in FIG. 4.

    R.sub.D2 I.sub.F2 ≧V.sub.3 ...                      (2.12)

On an external fault, an error voltage V_(E) appeared on thedifferentially connected CT secondary circuits is applied to the CT ofthe terminal at which the fault current flows out, and the error voltageV_(E) appearing across the differential circuit is as follows.

    V.sub.D =V.sub.E =(R.sub.S +R.sub.L)I.sub.F

On a light external fault, the differential relay does not operate dueto the following conditions. ##EQU8##

In the case of a heavy external fault, the differential relay does notoperate due to the following conditions. ##EQU9##

The following explains an example of the arrangement of the foregoingdifferential relay 87 in connection with FIG. 5. In FIG. 5, indicated by20-1 and 20-2 are terminals connected to the differential circuit 20, 21is a voltage transformer which transforms the voltage across theterminals into a proper voltage level, 35 is a harmonic contentsdiscrimination circuit which receives the secondary voltage of thevoltage transformer 21 and compares the second harmonic component (andother even-order harmonic components if necessary) in the differentialcircuit voltage with its fundamental component, 22-1 and 22-2 areignition circuits which receive the output of the harmonic contentsdiscrimination circuit 35 to produce a voltage having a proper magnitudeand pulse width and turn on or off semiconductor power switches of thefollowing stage, and 23-1 and 23-2 are the semiconductor power switcheswhich are triggered in response to the outputs of the ignition circuits22-1 and 22-2 thereby to open or close voltage waves of complementarypolarities, and are formed of GTO or SIT devices that becomesnonconductive by being triggered. Indicated by 24 is a resistor with itsboth ends being opened or short-circuitted by the semiconductor powerswitches 23-1 and 23-2, 25 is a resistor connected in series to theresistor 24, 26 is a current transformer having its primary windingconnected in series to the resistors 24 and 25, and 27 is a voltagetransformer which couples the voltage across the resistor 25 through itssecondary windings to ignition circuits 28-1 and 28-2, which thencontrol semiconductor power switches 29-1 and 29-2 (including SCRs whichbecome conductive by being triggered) to latch.

Indicated by 30 is a voltage detecting relay element connected to atertiary winding of the voltage transformer 21 and it responds to themagnitude of a voltage across the terminals 20-1 and 20-2. 31 is avoltage detecting relay element which detects a voltage transformed bythe voltage transformer 26 and operates when a primary current of thevoltage transformer 26 has reached a predetermined value in terms of thevoltage between the terminals 20-1 and 20-2 thereby to generate a "1"signal (to close contacts). 32 is a harmonic detecting relay elementwhich detects even harmonic components (particularly, the secondharmonic) in the voltage transformed by the voltage transformer 26 andgenerate a "1" signal (to close contacts). 33 is a voltage detectingrelay element which detects the magnitude of a primary voltage (current)transformed by the voltage transformer 27, and it operates when the CTsecondary currents flowing from the terminals 20-1 and 20-2 are largerthan a predetermined value thereby to form a proper short circuit so asto lower the impedance seen from the terminals 20-1 and 20-2 for thepurpose of high-speed operation.

FIG. 6 is a block diagram showing the arrangement of the harmonic ratiodiscrimination circuit 35. In FIG. 6, indicated by 41 and 42 are a2-terminal reactance filter which passes the fundamental component ofthe input between terminals 35-1 and 35-2 connected to the secondarywinding of the voltage transformer 21 and blocks the second harmoniccomponent, to output a passing current through the filter as a voltagedrop across a resistor 43. 44 and 45 are a 2-terminal reactance filterwhich passes the second harmonic component and blocks the fundamentalcomponent, to output a passing current through the filter as a voltagedrop across a resistor 46.

Indicated by 47-1 and 47-2 are insulating amplifiers for the outputvoltages of the resistors 43 and 46, and yield output voltages to phaseshift circuits of the following stage. 48-1 and 48-2 are phase shiftamplifying circuits which adjust the phase of the output voltages of theamplifiers 47-1 and 47-2 thereby to produce output voltages insynchronism with the frequency and harmonics of the power system. 49 isa current transformer which receives the outputs of the phase shiftamplifiers 48-1 and 48-2 as a primary input, and outputs a secondaryoutput D(V_(D)) proportional to the difference of the primary input witha positive and negative polarities separately from its center-tappedsecondary winding to terminals 50-1 and 50-2.

The voltage transformer 21, harmonic ratio discrimination circuit 35,ignition circuits 21-1 and 21-2, and semiconductor power switches 23-1and 23-2 in combination constitute a first switching over circuit 61which switches the impedance of the differential circuit in accordancewith the ratio of the even-order harmonic components to the fundamentalcomponent of the differential circuit input. Similarly, the voltagetransformer 27, ignition circuits 28-1 and 28-2, and semiconductor powerswitches 29-1 and 29-2 in combination constitute a second switching overcircuit 62 which switches the impedance of the differential circuit inaccordance with the terminal voltage of the differential circuitimpedance. The "0" or "1" (closed/open) signals produced by theoperation of detecting relay elements 30, 31, 32 and 33 are delivered toa logic sequence circuit which forms an interlock circuit 51 fortripping a breaker as shown in FIG. 7. In FIG. 7, indicated by 52 is alock-out relay for tripping breaker, 52-a through 52-c are auxiliarycontact sets of the lock-out relay 52 of and 30-a, 30-b, 31-a, 32-a, and33-a are contact sets of the detection relay elements shown in FIG. 5.

Next, the operation of the embodiment shown in FIG. 5 will be described.

(a) Light internal fault

On an internal fault, currents flow from the terminals 1 to n-1 toward afault point F, and the differential circuit input is produced to theterminals 20-1 and 20-2. In this case, the input to the voltagetransformer 21 includes relatively a little distortion, causing theharmonic ratio discrimination circuit 35 to produce a too small outputto fire the ignition circuits 22-1 and 22-2, and the semiconductor powerswitches 23-1 and 23-2 remain closed to keep the resistor 24short-circuitted. Accordingly, it is equivalently the insertion of arelatively low impedance RD₁ provided by the resistor 25 and currenttransformer 26 between the terminals 20-1 and 20-2, and consequently thedifferential relay operates as a low-impedance differential relay. Ifthe detecting relay element 31 operates (close contact 31a) and thedetecting relay element 30 does not operate (close contact 30b), thelock-out relay 52 is energized.

(b) Heavy internal fault

In this case, a large current is applied to the terminals 20-1 and 20-2,causing the ignition circuits 22-1 and 22-2 to produce large outputs,and the CT11 to CT(1n-1) of terminals 1 to n-1 are less likely to besaturated by the fault currents fed by the power source. Therefore, itseven harmonic components I_(D2), I_(D4), ..., I_(D2n) are small and theharmonic ratio discrimination circuit 35 produces a large outputD(V_(D)). This output provides a sufficiently large inputs for theignition circuits 22-1 and 22-2, causing the semiconductor powerswitches 23-1 and 23-2, which receive the outputs of 22-1 and 22-2, toopen anode circuit, and the resistor 25 is inserted in series to theresistor 24. Consequently, a series circuit made up of the resistors 24and 25 and current transformer 26 has a high impedance, the impedanceseen from the terminals 20-1 and 20-2 becomes also high, and thedifferential relay operates as a high-impedance differential relay.

In this case, large currents flow in from the CTs heavy and largevoltage drop appears across the resistor 25, and the voltage applied tothe voltage transformer 27 is also high. Consequently, the voltagetransformer 27 produces an output large enough to fire the ignitioncircuits 28-1 and 28-2.

On receiving the outputs of the ignition circuits 28-1 and 28-2, thesemiconductor power switches 29-1 and 29-2 become conductive alternatelyin every half cycle to short-circuit the resistors 24 and 25. As aresult, the internal impedance seen from the terminals 20-1 and 20-2becomes sufficiently low and the voltage of the differential circuitdoes not rise in excess, and it does not impairs the insulation of theCT secondary circuits connected to the differential circuit.

(c) Light external fault

Among the CTs connected differentially, the CT connected to theterminals where the external fault current flow out see the sum of thevoltage drops along the secondary leads of CTs connected to theterminals where the fault currents are fed from the source behind. Sincethe fault current is small in this case, the voltage across the CTsecondary lead wires are low and the voltage applied between theterminals 20-1 and 20-2 is also low. Accordingly, the harmonic ratiodiscrimination circuit 35 produces a small output D(V_(D)), applicationof this small output upon the ignition circuits 22-1 and 22-2 cannotturn off the semiconductor power switches 23-1 and 23-2, and theresistor 24 is left short-circuitted. Therefore the differential relayoperates as a low-impedance differential relay connected in series withthe resistor 25 and current transformer 26. In this case, the outputV_(D) of harmonic ratio discrimination circuit 35 is small, causing thedetection relay element 30 to be reset (closed contact 30b) and thedetecting relay elements 31 and 32 to be also reset (open contacts 31aand 32a), and the lock-out relay 52 is not energized.

(d) Medium external fault

Similar to the above case, a voltage drop proportional to the sum of thecurrents flowing from the terminals where fault currents flow arisesbetween the terminals 20-1 and 20-2, and this voltage drop is greaterthan the case of the preceding light external fault. The detecting relayelement 30 may operate depending on the magnitude of the resultingdifferential circuit voltage V_(D). Depending on the instant theoccurrence of the fault, a d.c. component will appear, causing the CT ofthe terminal, from which the fault current flows out, to be d.c.saturated, and even harmonic components including the second harmonicare yielded in the excitation current, i.e., differential current. Inresponse to the even harmonic components, the detecting relay element 31stays reset due to its even harmonic discrimination and suppressingcharacteristics. Due to the reduction of the output with the existenceof the even harmonic bias mentioned before, the harmonic ratiodiscrimination circuit does not increase its output directionallyproportional to the magnitude of differential currents, but it producessufficient output only when the fault current input applied to 30 islarger enough for the pick-up of 30, thus firing the ignition circuit22-1 and 22-2. Then, the semiconductor power switches 23-1 and 23-2 arebrought open and the resistors 24 and 25 are inserted in series to thecircuit.

Namely, in contrast to the case of no harmonic ratio discriminationcircuit 35 used (i.e., preceding patent application), the differentialrelay operates as a high-impedance differential relay only after thepassage of a larger current. In other words, it attains a wider currentrange in which the inventive differential relay can operates as alow-impedance one than that of the preceding patent application. Itoperates as a low-impedance differential relay even in the range ofmedium external fault currents and never bring an unnecessary high valuein impedance of the CT secondary circuit.

(e) Heavy external fault

With a large through fault current, the detecting relay elements 30 and31 may operate. In common with the above case, the harmonic ratiodiscrimination circuit 35 also produces a large output, firing theignition circuits 22-1 and 22-2 to produce sufficient outputs. Then, thesemiconductor power switches 23-1 and 23-2 become open, and thedifferential relay operates as a high-impedance differential relay.Based on the operation as a high-impedance differential relay, bysetting the detecting relay element 32 as sufficiently high relative tothe differential circuit voltage V_(D) appearing across the CT secondarylead wire, the element 32 is prevented from malfunctioning, as is knownin the prior art.

The foregoing embodiment is an example of carrying out the presentinvention. Various other arrangements are possible, and they are,needless to say, included in the scope of this invention in its broadestaspect.

As described above, the inventive apparatus is arranged such that theimpedance of the differential circuit is switched over in response tothe terminal voltage of the differential circuit or the terminal voltageacross the impedance so that the differential relay is operated in thehigh-impedance differential relay mode or low-impedance differentialrelay mode, whereby the advantages of these modes can be usedcomplementally while avoiding their drawbacks.

Namely, differential protective relay apparatus based on one aspect ofthis invention is designed to detect even harmonic components in thedifferential circuit current which is observed in the event of anexternal fault and to switch over the impedance of the differentialcircuit in accordance with the contents of harmonics thereby to alterthe switching over voltage, whereby the differential circuit impedancecan be kept lower over a wide fault current range. In consequence, thedifferential relay has enhanced pickup sensitivity to internal faultdetection.

The differential protective relay apparatus based on another aspect ofthis invention is designed to switch over an impedance of thedifferential circuit to a lower impedance in accordance with theterminal voltage across the impedance, whereby the voltage of thedifferential circuit does not rise in excess and it does not impair theinsulation of the CT secondary circuits connected to the differentialcircuit.

What is claimed is:
 1. A differential protective relay apparatuscomprising: a plurality of terminals connected to a bus bar; currenttransformers each provided for said terminals respectively; adifferential circuit connected in parallel to secondary windings of saidcurrent transformers; a first switching over circuit for switching overan impedance of said differential circuit based on a defined functionalvalue determined by a fundamental component and even harmonic componentsof an input applied to said differential circuit; a plurality of voltagedetecting elements each for detecting a terminal voltage of saiddifferential circuit and a terminal voltage of said impedance; and abreaker tripping interlock circuit, including logically connectedcontacts to be made and broken by said voltage detecting elements, fortripping a breaker.
 2. A differential protective relay apparatusaccording to claim 1, wherein said impedance of said differentialcircuit is formed out of a plurality of resistors and a primary windingof a voltage transformer connected in series.
 3. A differentialprotective relay apparatus according to claim 2, wherein said firstswitching-over circuit comprises: a voltage transformer which transformsthe differential circuit input; a harmonic ratio discrimination circuitwhich is connected to a secondary winding of said voltage transformer,discriminates a proportion of even harmonic components of thedifferential circuit input and produces a signal in response to theexcess of said proportion over a predetermined value; an ignitioncircuit which produces an ignition signal in response to the outputsignal from said discrimination circuit; and a first switching deviceconnected in parallel to a first resistor among said plurality ofseries-connected resistors, for releasing said first resistor fromshort-circuiting in response to said ignition signal in order to switchover said impedance of differential circuit from a first impedance to ahigher second impedance in response to said output signal from saiddiscrimination circuit.
 4. A differential protective relay apparatusaccording to claim 3, wherein said plurality of voltage detectingelements comprises: a first detection relay element connected to atertiary winding of said voltage transformer of said first switchingcircuit; a second detection relay element connected in parallel to thesecondary winding of said voltage transformer whose primary winding isconnected in series to said plurality of resistors; a third detectionrelay element which discriminates the even-order harmonics; and a fourthdetection relay element which is connected in parallel with the secondresistor among said plurality of resistors and is responsive to avoltage across said second resistor or a current flowing in said secondresistor.
 5. A differential protective relay apparatus according toclaim 3, wherein said harmonic ratio discrimination circuit comprises: afirst resonance circuit which passes the fundamental component of thedifferential circuit input and blocks the even harmonic componentsthereof; and a second resonance circuit which blocks said fundamentalcomponent and passes said even harmonic components, said thresholdcircuit for outputting a signal which is proportional to a differencebetween the fundamental component and even harmonic components of thedifferential circuit input.
 6. A differential protective relay apparatusaccording to claim 4, wherein said interlock circuit comprises: a firstlogical circuit which connects a normally-closed contact of said firstdetecting relay element in series to a normally-open contact of saidsecond detection relay element; a second logical circuit which connectsa normally-open contact of said first detecting relay element in seriesto a normally-open contact of said third detecting relay element; and athird logical circuit which is operated to open and close by anormally-open contact of said fourth detecting relay element, said firstthrough third logical circuits being connected in parallel with eachother.
 7. A differential protective relay apparatus according to claim6, wherein said interlock circuit is connected in series to a lock-outrelay for tripping a breaker, and to a normally-closed contact of saidlockout relay.
 8. A differential protective relay apparatus according toany of claims 1 through 7 further comprising a second switching overcircuit which switches over said impedance of said differential circuitbased on the terminal voltage of said impedance.
 9. A differentialprotective relay apparatus according to claim 8, wherein said secondswitching over circuit comprises: a voltage transformer, connected inparallel to said second resistor, for transforming a voltage across saidsecond resistor; a ignition circuit, connected to a secondary winding ofsaid current transformer, for producing an ignition signal in responseto the second resistor voltage; and a switching device, connected inparallel to said series-connected resistors, for short-circuiting saidplurality of resistors in response to said ignition signal, said secondswitching over circuit for switching over to a third impedance lowerthan said first impedance by short-circuiting said series-connectedplurality of resistors upon detecting that said voltage across saidsecond resistor exceeds a predetermined value.